The invention disclosed here consists of a technique and apparatus for non-invasively measuring the concentration of blood analytes. Specifically, the technique enhances the accuracy of non-invasive optical spectroscopic blood analysis by rejecting the component of the diffusely reflected or transmitted optical signal which has not interacted with the blood. The invention is particularly suitable for home blood glucose testing but is applicable to other blood analytes and measurement setting (e.g. GP's office, hospital bedside or other hospital ward).
Blood is routinely analyzed during a wide range of medical diagnostic procedures. The concentration of certain biomolecules, such as glucose, urea, lactate and cholesterol provide important indicators of health. Blood samples are usually taken either using a venous puncture, if the sample is taken by a doctor or nurse, or using a spring loaded finger prick, if the sample is taken by the donor.
Physical withdrawal of a blood sample has a number of drawbacks:
risk of infection to the donor and others who are in contract with the donor, blood sample or disposables (e.g. swabs) used during the withdrawal; PA1 inconvenience and embarrassment, particularly in the case where the donor draws their own blood; PA1 pain associated with skin puncture on a sensitive body site; PA1 risk of permanent tissue damage, such as the formation of callouses on fingers used repeatedly for withdrawals. PA1 (1) weak absorption of glucose at these wavelengths, compared to the absorption of water and other tissue constituents; PA1 (2) low concentrations of glucose in the blood (typically 3 mM to 7 mM); PA1 (3) low volume fraction of blood in tissue (typically less than 10%).
A non-invasive technique--one which does not require a blood sample to be withdrawn--for determining the concentration of blood analytes would overcome most if not all of the drawbacks listed above.
Previous approaches to non-invasive blood analysis have largely used optical spectroscopic techniques to penetrate the blood just below the tissue surface. The most successful and widespread of these techniques is pulse oximetry, which is used to determine blood oxygenation. However, more recently, effort has been focused on determining the concentration of blood glucose. This analyte is of particular interest because of the large home blood glucose testing market.
Non-invasive blood glucose determination has frequently used near infra-red radiation (wavelengths from 700 nm to 2500 nm) because of its ability to penetrate several millimeters into body tissues. However, diffuse reflectance or transmission measurement are complicated by the:
Previous work in this area has used multi-variate spectral analysis techniques (e.g. partial least squares and principal components regression) in an attempt to overcome the complicating factors listed above. However, the accuracy which can be achieved by multi-variate analysis is hardly sufficient for glucose determination and it is desirable that the accuracy of this approach is increased.
The invention disclosed here allows the accuracy of non-invasive optical spectroscopic blood analysis to be increased by analyzing only the component of the diffusely reflected or transmitted optical signal which has interacted with the blood. The approach is fully compatible with near infra-red spectroscopy and multi-variate analysis.